8 When cells divide, you need to have 2 copies of the DNA in the cellDNA Replication

9 DNA ReplicationMost important feature of DNA is its ability to replicate itself, (almost) exactlyThe double helix is ‘unzipped’ during replicationNew nucleotides are aligned along the DNA strands (built by DNA polymerase)The new helix contains one original strand and one new strand

13 The structure of ribonucleic acidLearning Objectives:Describe the structure of ribonucleic acid (RNA)Describe the structure and role of messenger RNA (mRNA)Describe the structure and role of transfer RNA (tRNA)

16 Messenger RNA (mRNA)mRNA is used to transfer the DNA code from the nucleus to the cytoplasm.mRNA is complementary to the DNA code.mRNA is small enough to leave the nuclear pores.Once in the cytoplasm, mRNA associates with the ribosomes.mRNA is used to determine the sequence of amino acids during protein synthesis.mRNA is easily broken down and only exists whilst it is needed to manufacture a protein.The sequence of nucleotides in the mRNA is referred to as the genetic code.

18 The Genetic Code (recap of unit 1)3 nucleotide bases on mRNA (a codon) codes for each amino acid in a protein.A few amino acids have a single codon.The code is degenerate – most amino acids have more than one codon.3 codons code for no amino acids – they are stop codons and mark the end of the polypeptide chain.Non-overlapping – each nucleotide is read only once.Universal – same codon codes for the same amino acid in all organisms (few minor exceptions).

19 Transfer RNA (tRNA) tRNA is a small molecule (~80 nucleotides).Single stranded and folded into a clover leaf shape with one end of the chain slightly longer.This longer section is attached to an amino acid.Each tRNA can carry a different amino acid.3 bases at the opposite end of the tRNA are called an anticodon.Each amino acid has a different anticodon.The anticodon pairs with the complementary codon on the mRNA.

25 TranscriptionHelicase (enzyme) acts on a region of DNA to break the hydrogen bonds between the bases.RNA polymerase moves along one of the two DNA strands – template strandRNA polymerase matches up complementary RNA nucleotides.C matches G and G matches CU matches A and A matches TAs the RNA nucleotides join together the pre-mRNA is formed.The DNA behind the RNA polymerase rejoins into a double helix.When the RNA polymerase reaches a “stop” codon, the chain is terminated and the pre-mRNA detaches.

26 Splicing of mRNA to form mRNAOnce the introns (non-functional DNA) have been removed, the remaining exon sections join together.

28 Fun fact of the day…Introns figured prominently in Star Trek: The Next Generation, Season 7, Episode 271, entitled "Genesis."In this episode, a synthetic T-cell inadvertently activated the crewmembers' introns, resulting in de-evolution and the expression of ancient physiological traits

32 Synthesising the polypeptide (translation)mRNA attaches to the ribosome at the “start” codon – AUG.tRNA with a complementary anticodon attaches to the mRNA – UAC.This tRNA is attached to an amino acid – methionineThe ribosome moves along the mRNA bringing in 2 tRNA molecules at any one time.An enzyme and ATP are used to join the amino acids with a peptide bond.The first tRNA is released and can collect another amino acid.The process is repeated until a “stop” codon is reached.Many ribosomes can travel along the mRNA at the same time – polysome.

35 TASK Using PCs log onto www.WolframAlpha.comEnter sequence of 22 bases from an imaginary DNA coding strandTry several different strands with same software.Who can get the most matches to the human genome?

36 TRANSLATION or TRANSCRIPTIONTASKCreate a model of eitherTRANSLATION or TRANSCRIPTIONCan be any format e.g.: PowerPoint, Poster, 3D Model